HD 221585 b
Astrophysical Synthesis & Habitability Characterization
HD 221585 b is a confirmed gas giant exoplanet orbiting the star HD 221585, located approximately 182.2 light-years (55.85 parsecs) from the Solar System. First announced in 2016 by Haute-Provence Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.5 Earth radii (1.2 Jupiter radii) and has a measured mass of 511.706 Earth masses (1.61 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.15 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 2.81 g⊕ (27.56 m/s²), and an escape velocity of 68.9 km/s.
In its orbital architecture around HD 221585, HD 221585 b completes one full revolution every 1173 Earth days (3.21Julian years) at a semi-major axis of 2.306 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 21.4 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.123 carries the world between 2.0224 AU at periapsis and 2.5896 AU at apoapsis, driving measurable seasonal insolation cycles.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 13.500 R⊕, HD 221585 b exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star HD 221585 is classified as a solar-type yellow star of spectral type G8 IV with an effective photospheric temperature of 5,620 K, a radius of 1.8506 R☉, a mass of 1.19 M☉, an estimated bolometric luminosity of 3.065 L☉. Stellar evolution modeling indicates an estimated stellar age of 6.2 Gyr and a metallicity of [Fe/H] = 0.29 dex (metal-rich relative to the Sun). From Earth, HD 221585 shines at apparent V-band magnitude 7.457 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: Haute-Provence Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 221585 |
| Spectral Type | G8 IV |
| Effective Temp (Teff) | 5,620 K |
| Stellar Radius | 1.8506 R☉ |
| Stellar Mass | 1.1900 M☉ |
| Bolometric Luminosity | 3.065 L☉ |
| Metallicity [Fe/H] | 0.290 dex |
| Stellar Age | 6.200 Gyr |
| Apparent Magnitude | V: 7.457 · G: 7.281 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 2.306000 AU |
| Orbital Period | 1173 days |
| Mean Orbital Velocity | 21.4 km/s |
| Orbital Eccentricity (e) | 0.12300 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.15 g/cm³ (68.9 km/s vesc) |
| Insolation Flux | 0.4965 S⊕ (~676 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 182.200 LY (55.854 pc) |